It is known in the art to provide, in paper handling apparatus such as printers, web transport devices which include a rotatably mounted and driven friction roll and a plurality of rotatable pressure rollers arranged individually on and along the circumference of the friction roll. Such devices are commonly employed for the drawing in or pull-type feeding of a variety of typically paper-based documents or recording media as, for example, vouchers that are used in banks, industrial concerns, insurance companies and similar businesses. The device can, accordingly, form a part of a printer, a voucher processing device or any of a variety of other office machines.
It is also known, in devices of these types, to determine the lateral position of the recording medium by way of stops for contact with both of the opposed lateral edges of the paper or other recording medium. In constructions in which these stops are fixed in position, the device is suitable for only a single specific width of recording medium, the accuracy and position of the recording medium being decisively dependent on the tolerances of the recording medium width. These tolerances may easily, and commonly do, exceed 1% of the width of the recording medium due, at least in part, to both manufacturing tolerances and variations in temperature and humidity.
Where, on the other hand, one of the laterally-opposed stops is displaceable, several or a range of widths can be accomodated and processed through the apparatus. In such arrangements, however, certain difficulties nevertheless arise if a pressure roller should happen to lie on the side or lateral edge of the recording medium. In that case, the forces on the recording medium result in lateral travel or movement of the medium and thereby lead to disturbances in operation. Moreover, the provision of an adjustable stop means or element constitutes an additional expense in design and manufacture of the device. It must additionally be recognized that the operator must have the ability to suitably and correctly adjust the movable stop upon each and any change in width of the recording medium.
Still another difficulty arises where the recording medium or paper is initially inserted into the device in an oblique or unduly angled orientation; this further increases the danger that the edge of the recording medium will become damaged during feeding or transport and thereby negatively impact general operation of the device.
A second heretofore known effort to solve these problems lies in the provision of rubber-covered conical pressure rollers by means of which lateral components of movment of the recording medium are produced to move the medium along a reference edge. In this case, however, the frictional forces, and thus the transmission of force to the recording medium, must be via the pressure rollers and not (or only in part) via the underlying friction roll. As a consequence, the pressure rollers must also be driven which, as will be appreciated, adds substantial additional expense from a structural and manufacturing standpoint. Furthermore, in such constructions the adherence of the recording medium to the friction roll must be less than the medium's adherence to the driven pressure rollers in order to permit lateral movement of the recording medium during its transport.
In a third known arrangement for determining the lateral position of a transported recording medium, pressure rollers which are swingable through a sharply acute angle guide the recording medium along a reference edge. In this case the pressure rollers must be separately driven and covered with a rubber surface. These pressure rollers must also be swung or pivoted between two, relatively opposed positions for accomodating paper transport in either of two, e.g. forward and reverse, respective directions. Thus, this arrangement is also quite expensive to structurally implement and manufacture.